1. Field of the Invention
The present invention relates to optic techniques, and more particularly to automatic focusing.
2. Description of the Related Art
Autofocusing in a camera is accomplished by focus adjustment according to measured distances between the subjects and the camera utilizing a range finder. Passive autofocusing utilized in digital cameras eliminates range finders. Digitized images are captured through the lens, and clarity values thereof are obtained by frequency domain transformation and treated as the basis for focus adjustment in which an image with maximum clarity is found.
A lens, referred to as focusing lens, in a conventional camera is driven by a motor to 15-20 positions for focus adjustment. For example, assume that a focusing lens of a camera is movable to N positions. In FIG. 1, coordinates of every point on the horizontal axis represent positions of the focusing lens, and coordinates on the vertical axis are clarity values. After clarity values corresponding to each position of the focusing lens have been obtained, the focusing lens is adjusted to a position PF corresponding to the maximum clarity value Mmax.
Current focusing lenses can be driven from 20 to 200 positions. Executing frequency domain transformation for each position, however, is very time consuming. Focus adjustment, therefore, is divided into two stages. First, clarity values corresponding to a portion of N positions are computed. For example, as shown in FIG. 2, results of frequency domain transformation corresponding to seven positions are derived, wherein M corresponding to position P is the current relative maximum value. As shown in FIG. 3, in the second stage, clarity values corresponding to positions around position P are calculated in detail to locate the absolute maximum value Mmax corresponding to position PF.
Two stage autofocusing, however, is still time consuming in practice.